2-22
Class AB Amplifier Operation
Amplifiers designed for class AB operation are biased so that collector current is zero (cutoff) for a
portion of one alternation of the input signal. This is accomplished by making the forward-bias voltage
less than the peak value of the input signal. By doing this, the base-emitter junction will be reverse biased
during one alternation for the amount of time that the input signal voltage opposes and exceeds the value
of forward-bias voltage. Therefore, collector current will flow for more than 180 degrees but less than 360
degrees of the input signal, as shown in figure 2-15 view B. As compared to the class A amplifier, the dc
operating point for the class AB amplifier is closer to cutoff.
The class AB operated amplifier is commonly used as a push-pull amplifier to overcome a side effect
of class B operation called crossover distortion.
Class B Amplifier Operation
Amplifiers biased so that collector current is cut off during one-half of the input signal are classified
class B. The dc operating point for this class of amplifier is set up so that base current is zero with no
input signal. When a signal is applied, one half cycle will forward bias the base-emitter junction and IC
will flow. The other half cycle will reverse bias the base-emitter junction and IC will be cut off. Thus, for
class B operation, collector current will flow for approximately 180 degrees (half) of the input signal, as
shown in figure 2-15 view C.
The class B operated amplifier is used extensively for audio amplifiers that require high-power
outputs. It is also used as the driver- and power-amplifier stages of transmitters.
Class C Amplifier Operation
In class C operation, collector current flows for less than one half cycle of the input signal, as shown
in figure 2-15 view D. The class C operation is achieved by reverse biasing the emitter-base junction,
which sets the dc operating point below cutoff and allows only the portion of the input signal that
overcomes the reverse bias to cause collector current flow.
The class C operated amplifier is used as a radio-frequency amplifier in transmitters.
From the previous discussion, you can conclude that two primary items determine the class of
operation of an amplifier — (1) the amount of bias and (2) the amplitude of the input signal. With a given
input signal and bias level, you can change the operation of an amplifier from class A to class B just by
removing forward bias. Also, a class A amplifier can be changed to class AB by increasing the input
signal amplitude. However, if an input signal amplitude is increased to the point that the transistor goes
into saturation and cutoff, it is then called an OVERDRIVEN amplifier.
You should be familiar with two terms used in conjunction with amplifiers — FIDELITY and
EFFICIENCY. Fidelity is the faithful reproduction of a signal. In other words, if the output of an
amplifier is just like the input except in amplitude, the amplifier has a high degree of fidelity. The
opposite of fidelity is a term we mentioned earlier — distortion. Therefore, a circuit that has high fidelity
has low distortion. In conclusion, a class A amplifier has a high degree of fidelity. A class AB amplifier
has less fidelity, and class B and class C amplifiers have low or "poor" fidelity.
The efficiency of an amplifier refers to the ratio of output-signal power compared to the total input
power. An amplifier has two input power sources: one from the signal, and one from the power supply.
Since every device takes power to operate, an amplifier that operates for 360 degrees of the input signal
uses more power than if operated for 180 degrees of the input signal. By using more power, an amplifier
has less power available for the output signal; thus the efficiency of the amplifier is low. This is the case
